After Hans discovered some issues I tested them one more time - and yes, he is right:
1. (left folding chair) This issue is no problem for me, but it would be nice if we can turn the whole chair standing upright.
2. (right folding chair) Same problem as in point 1 but in a different position.
3. (right folding chair) The hinges of the back legs are "Broken". They move out of the counterpart...